Duplex Stainless Steel Having Excellent Corrosion Resistance with Low Nickel

ABSTRACT

Disclosed is duplex stainless steel that containes relatively low content of Ni, and limits constituents of Cr—Mo—Mn—N to make volume fraction of α and γ have about 50:50, thereby minimizing incidence of a edge crack to enhance a production yield and decrease a processing load, in which the alloy constituents includes Cr of 19.5˜22.5%. Mo of 0.5-2.5%, Ni if 1.0-3.0%, Mn of 1.5-4.5%, N of 0.15-0.25%, Fe and unavoidable elements, and a constitution range of the alloy constituents are adjusted to make a CPt higher than 20° C. depending on the constitution range of the alloy constituents. Thus, the contents of Cr, Mo and Ni is decreased and the content of Mn is increased a little, so that a production cost thereof is reduced; the corrosion resistance is secured to be better than the STS 304 steel and the 316L steel; the incidence of the edge cract is decreased while being hot-rolled, thereby decreasing a load on the following process; and the surface defective is decreased, thereby improving a production yield.

BACKGROUND ART

1. Field of the Invention

The present invention relates to duplex stainless steel containing Mn ofhigh content and Cr, Mo, N and Ni of low contents as compared withS32205 duplex stainless steel, and more particularly, to duplexstainless steel that includes low contents of Cr, Mo, N and Ni tothereby decrease a production cost thereof, has excellent corrosionresistance better than STS 304 steel and 316L steel, and has a lowincidence of an edge crack when it is hot-rolled.

2. Description of Related Art

In general, austenite stainless steel excellent in formability andcorrosion resistance uses Fe as base metal and mainly contains Cr andNi. Further, the austenite stainless steel has been variously developedby adding other elements such as Mo, Cu, etc. for various purposes.

Among the austenite stainless steel, 316L steel is excellent incorrosion resistance, pitting resistance and high temperature strength.However, the 316L steel is low carbon steel and contains Ni more thanlOwt% and Mo more than 2wt%, so that a cost price thereof heavilyfluctuates according to the price of Ni and Mo, thereby decreasingcompetitive power.

To increase the competitive power, iron and steel business tries todevelop new steel by lowering contents of Ni and Mo and securingcorrosion resistance better than that of the 316L steel.

As an example of duplex stainless steel that has mixed formation of anaustenite phase and a ferrite phase, S32205 duplex stainless steel(hereinafter, referred to as “2205 steel” contains high percentage ofCr, Mo and N to secure excellent corrosion resistance, and contains Nimore than 5wt% to secure a volume fraction.

Such duplex stainless steel contains a relatively low percentage of Nias compared with STS 316L steel containing 10% Ni, so that itsproduction cost is low and thus its price is competitive, therebyincreasing added value. However, the 2205 steel has poor hot-formabilityand thus has a very low production yield of 80% . Further, the 2205steel has high contents of Cr and Mo, so that a sigma-phase depositionrate is high, thereby deteriorating the property of steel and having ahigh load on winding and cooling processes. Thus, it is hard to replacethe 316L steel by the 2205 steel.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide duplexstainless steel that has low contents of Cr, Mo, N and Ni as comparedwith 2205 steel to thereby decrease a production cost thereof, increasesproduction yield by lowering an incidence of an edge crack when it ishot-rolled, and has excellent corrosion resistance better than STS 304steel and 316L steel.

The present inventor develops duplex stainless steel that containsrelatively low content of Ni, and limits constituents of Cr-Mo-Mn-N tomake volume fraction of a and y have about 50:50, so that a productioncost is reduced; a CPT is secured to be higher than 20° C. of that ofthe STS 304 steel and the 316L steel; and the incidence of a edge crackis minimized to enhance a production yield and decrease a processingload.

In an exemplary embodiment of the present invention, duplex stainlesssteel includes Cr of 19.5-22.5%, Mo of 0.5-2.5%, Ni of 1.0-3.0%, Mn of1.5-4.5%, N of 0.15%-0.25%, C of 0.03% and less, P of 0.03% and less, Siof 2% and less, Fe and un-avoidable elements.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other objects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe preferred em- bodiments, taken in conjunction with the accompanyingdrawings of which:

FIG. 1 are photographs showing an edge crack of a hot-rolled sampleaccording to alloy constituents;

FIG. 2 is a table showing alloy constituents and a volume fraction ofsteel samples according to the present invention and comparative steelsamples;

FIG. 3 is a table showing critical pitting temperatures (CPT) of thesamples of FIG. 2;

FIG. 4 is a table showing total test results of the samples of FIG. 2with regard to corrosion resistance and hot formability; and

FIG. 5 is a table showing oxidation increment of the steel according tothe present invention and the conventional steel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to accompanying drawings. Below, essential constituents arelimited as follows.

Carbon (C): C is effective for strengthening a material by solidsolution strengthening. However, when the content of C is excessive, Cis easily combined with a carbide-forming element such as Cr, which iseffective for corrosion resistance in a boundary betweenferrite-austenite phases. Thus, C lowers the content of Cr around agrain boundary, so that the corrosion resistance is deteriorated. Tomaximize the corrosion resistance, the content of C is lowered into0.03% and below.

Nitrogen (N): N, together with Ni, is one of elements that contributestabilization of an austenite phase. As the content of N increases, thecorrosion resistance and high strengthening are achieved. However, whenthe content of N is too high, the hot formability of steel isdeteriorated, thereby lowering the production yield thereof. On theother hand, when the content of N is too low, the contents of Cr and Moshould be lowered to secure the volume fraction of steel, and it isdifficult to secure the strength of a welding part and phase stability.Therefore, the content of N preferably ranges between 0.15% and 0.25%.

Manganese (Mn): Mn generally has a content of about 1.5% to adjust ametal flow rate. In the meanwhile, Mn can be additionally containedinstead of Ni. In this case, the hot formability can be secondarilyimproved. However, when the content of Mn is excessive, Mn is combinedwith S of the steel and forms MnS, thereby deteriorating the corrosionresistance and the hot formability. Thus, the upper limit content of Mnis limited to 4.5%. Preferably, the content of Mn ranges between 1.5%and 4.5%.

Chrome (Cr): Cr, together with Mo, is used as an element to stabilizethe ferrite phase. Here, Cr is essential to not only primarily securingthe ferrite phase of duplex stainless steel but also securing excellentcorrosion resistance. When the content of Cr increases, the corrosionresistance increases, but the content of expensive Ni should be alsoincreased to maintain the volume fraction. In results, the content of Cris preferably limited between 19.5% and 22.5% so as to maintain thevolume fraction of t he duplex stainless steel and the corrosionresistance better than that of STS 304 and 316L steel.

Molybdenum (Mo): like Cr, Mo is used for not only stabilizing theferrite phase but also largely enhancing the corrosion resistance.However, if the content of Mo is excessive, Mo is likely to form thesigma phase when it is annealed, thereby dete- riorating the corrosionresistance and impact resistance. In the present embodiment, Mo justassists Cr in securing the ferrite volume fraction and secures theproper corrosion resistance, so that the content of Mo is preferablylimited between 0.5% and 2.5%.

Nickel (Ni): Ni, together with Mn and N, is an element to stabilize theaustenite phase, and mainly used in securing the austenite phase of theduplex stainless steel. To reduce a production cost, if the content ofexpensive Ni is lowered, the decrement of Ni can be replaced byincreasing the content of Mn and N that form the austenite phase.However, if the content of Ni is excessively lowered, Mn and N should beex- cessively needed so that the corrosion resistance and the hotformability are de- teriorated, or the contents of Cr and Mo are loweredso that it is difficult to secure the corrosion resistance better thanthe 316L steel. Thus, the content of Ni preferably ranges between 1.0%and 3.0%.

Phosphorous (P): P is seeded in the grain boundary or an interface, andis likely to deteriorate the corrosion resistance and toughness.Therefore, the content of P is lowered as low as possible. Preferably,the upper limit content of P is limited to 0.03% in consideration of theefficiency of a refining process.

Sulfur (S): S deteriorates the hot formability, or forms MnS togetherwith Mn, thereby deteriorating the corrosion resistance. Therefore, thecontent of S is lowered as low as possible. Preferably, the content of Sis lower than 0.03%.

Silicon (Si): Si is added for deoxidization, but it can act as anelement for stabilizing the ferrite phase. If the content of Si isexcessive, Si deteriorates the mechanical property such as impacttoughness of steel. Therefore, the content of Si is preferably limitedto 2% and below.

Meanwhile, samples of duplex stainless steel having constituentsaccording to an embodiment of the present invention are prepared andthey are tested about the volume fraction, the corrosive resistance andthe hot formability. FIG. 2 shows alloy con- stituents of the samplesand a-volume fractions after they are annealed at a temperature of 1050°C. In these alloys, the a-volume fractions thereof range from about 40to about 60%. Regarding welding, phase stability, and the like, it isdetermined that the duplex stainless steel has an a-volume fractionranging from 44 to 51% is excellent (0); the duplex stainless steel hasan a-volume fraction lower than 44% or higher than 54% is defective (X);and the duplex stainless steel has an a-volume fraction ranging from 51to 54% is good (A).

In the samples of FIG. 2, the alloy constituents except Cr, Mo, Mn and Nare unified to satisfy general content rages of the duplex stainlesssteel, but the content rage of Ni is limited to 2.5 wt% for experimentalconvenience.

FIG. 3 is a table showing critical pitting temperatures (CPT) of thesamples of FIG. 2, in which the CPT means the corrosion resistance.Here, it is determined that the steel having a CPT of 20° C. and belowis defective (X); the steel having a CPT ranging from 20° C. to 25° C.and below is good (?); and the steel having a CPT of 20° C and higher isexcellent (0).

The foregoing CPTs result from annealing the hot-rolled samples having asize of 50 mm(L)×25 mm(W)×3 mm(T) at a temperature of 1050° C. on thebasis of an American society for testing and materials (ASTM) G48method, and then depositing it in acidified ferric chloride solution for24 hours.

According to the ASTM G48 method, a CPT measuring starting temperatureis obtained by the following equation:

CPT(° C)=(2.5×%Cr)+(7.6×%Mo)+(31.9×%N)−41.0.

The ASTM G48 method suggests calculating the CPTs and selecting theclosest value at intervals of 5° C.

However, the CPT measuring starting temperature estimated by the ASTMG48 has a large deviation. Why the deviation is large is because thecorrosion resistance de- teriorated by Mn is not considered. That is,because the duplex stainless steel having lowered contents of Ni hasrelatively high content of Mn, the deviation arises in the estimated CPTobtained by the foregoing ASTM G48 method.

To compensate the deviation, the present inventor calculates the CPT byconsidering Mn as follows.

CPT(° C)=−150.47+2.65Cr+11.71Mo−1.3Mn+64.58N.

According to the present invention, the estimated CPT is approximatelyequal to the measured CPT.

Meanwhile, in the case of the steel having the low contents of Cr and Moor having the high content of Mn, the measured CPT is relatively lowerthan the estimated CPT.

Therefore, it is undesirable that the content of Mn is excessivelyincreased or the contents of Cr and Mo is excessively decreased in orderto secure the volume fraction of duplex stainless steel with reduced Ni.

While producing the duplex stainless steel with reduced Ni, theproduction yield should be increased to decrease the incidence of anedge crack, and the hot formability should be secured to minimize aninvariable load. The steal with constituents of FIG. 2 is produced as aningot of 50 kg and pressed to have 20 mm(T) and 30 mm(T), and then theincidence of their edge crack is observed, thereby getting test resultsas shown in FIG. 1 by selecting the steel remarkably improved in theincidence of the edge crack as compared with the 2205 steel.

Here, it is determined that the steel having the edge crack like the2205 steel is defective (X); the steel having a local edge crack is good(A); and the steel having little edge crack is excellent (0).

FIG. 4 shows total test results of the samples of FIG. 2 with regard tothe volume fraction, the corrosion resistance, and the hot formability.Four steels (steel Nos. 3, 4, 14, 15) satisfy the formation propertiesof the duplex stainless steel, has the corrosion resistance better thanthat of the 316L steel, and is excellent in the hot formability.

Further, other steels (steel Nos. 1, 2, 5, 6, 7, 8, 9, 10, 11, 12, 13,15, 16, 20, 21, and 22) can be selected as preferred steel, but they areinferior to the foregoing steels. Thus, the hot formability and thecorrosion resistance are deteriorated as the content of Mn becomeshigher. Further, the hot formability is deteriorated as the content of Nbecomes higher. Also, the steels having the high content of Mn needsrelatively higher content of Mo.

FIG. 5 shows difference in high temperature oxidation between theexcellent and good steels according to the embodiment of the presentinvention and comparative steels such as STS304, STS316L and 2205 whenthey are reheated as slabs for hot-rolling. T he high temperatureoxidation measurement is performed by measuring oxidation incrementunder the condition that the hot-rolled sample having a size of 10mm(L)×10 mm(W)×3 mm(T) is heated at a temperature of 1250° C. andremained in a heating furnace for 180 minutes.

At this time, under gas atmosphere of the heating furnace, the contentof S is adjusted into 200 ppm. In results, the oxidation increment ofthe steel according to the present invention is 4 through 6 times lowerthan the convention 2205 steel, and about ⅓ through ½ times higher thanthe 316L steel. As compared with the conventional 2205 steel, thesurface quality of the steel according to the present invention isenhanced as a surface defective is decreased by the surface lubricationeffect due to an oxidation layer formed on a surface of a reheating slabwhile being hot-rolled.

In the duplex stainless steel according to the present invention ascompared with the 2205 duplex stainless steel, the contents of Cr, Moand Ni is decreased and the content of Mn is increased a little, so thata production cost thereof is reduced; the corrosion resistance issecured to be better than the STS 304 steel and the 316L steel; theincidence of the edge crack is decreased while being hot-rolled, therebydecreasing a load on the following process; and the surface defective isdecreased, thereby improving a production yield.

Although a few embodiments of the present invention have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges might be made in this embodiment without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

1. (canceled)
 2. (canceled)
 3. A duplex stainless steel having excellentcorrosion resistance with low Ni, having alloy constituents comprisingCr ranging from 19.5 wt% to 22.5 wt%, Mo ranging from 0.5 wt% to 2.5wt%, Ni ranging from 1.Owt% to 3.0 wt%, Mn ranging from 1.5 wt% to 4.5wt%, N ranging from 0.15 wt% to 0.25 wt%, Fe and unavoidable elements,wherein a critical pitting temperature of the duplex stainless steel iscalculated as a function of constitution ranges of Cr, Mo, Mn, and N inthe alloy constituents, and the constitution ranges of the alloyconstituents are adjusted to have the critical pitting temperature of20° C. or more.
 4. The duplex stainless steel according to claim 3,further including C of 0.03 wt% or less P of 0.03 wt% or less, and Si of2 wt% or less.
 5. The duplex stainless steel according to claim 3,wherein the critical pitting temperature of the duplex stainless steelis calculated by:CPT=CONST+2.65Cr+11.71Mo−1.3Mn+64.58N, and wherein CPT is the criticalpitting temperature in ° C and CONST is a constant for calculating thecritical pitting temperature in C.
 6. The duplex stainless steelaccording to claim 5, wherein the constant, CONST, is a negativeconstant.
 7. The duplex stainless steel according to claim 6, whereinthe negative constant is −50.47.
 8. The duplex stainless steel accordingto claim 7, wherein the constitution ranges of the alloy constituentsare adjusted to have the critical pitting temperature of 25° C. or more.9. The duplex stainless steel according to claim 3, wherein theconstitution ranges of the alloy constituents are adjusted to have thecritical pitting temperature of 25° C. or more.
 10. A method for formingduplex stainless steel having excellent corrosion resistance with lowNi, having alloy constituents including Cr ranging from 19.5 wt% to22.5wt%, Mo ranging from 0.5 wt% to 2.5 wt%, Ni ranging from 1.0wt% to3.0 wt%, Mn ranging from 1.5 wt% to 4.5 wt%, N ranging from 0.15 wt% to0.25 wt%, Fe and unavoidable elements, the method comprising:calculating a critical pitting temperature of the duplex stainless steelas a function of constitution ranges of Cr, Mo, Mn, and N in the alloyconstituents, and adjusting the constitution ranges of the alloyconstituents to have the critical pitting temperature of 20 C or more.11. The method according to claim 10, further comprising: adding C of0.03 wt% or less, P of 0.03 wt% or less, and Si of 2 wt% or less to formthe duplex stainless steel.
 12. The method according to claim 10,wherein the calculating the critical pitting temperature of the duplexstainless steel comprises calculating the critical pitting temperatureof the duplex stainless steel using:CPT=CONST+2.65Cr+11.71Mo−1.3Mn+64.58N, and wherein CPT is the criticalpitting temperature in ° C and CONST is a constant for calculating thecritical pitting temperature in ° C.
 13. The method according to claim12, wherein the constant, CONST, is a negative constant.
 14. The methodaccording to claim 13, wherein the constant, CONST, is -50.47.
 15. Themethod according to claim 14, wherein the adjusting the constitutionranges of the alloy constituents comprises adjusting the constitutionranges of the alloy constituents to have the critical pittingtemperature of 25 C or more.
 16. The method according to claim 10,wherein the constitution ranges of the alloy constituents are adjustedto have the critical pitting temperature of 25 C or more.